A fragment based step-by-step strategy for determining the most stable conformers of biomolecules

Identifying the protonation site and the scope of non-proline cis-peptide bond conformations: a first-principles study on protonated oligopeptides

The existence of non-proline cis-peptide bond conformations of protonated triglycine proposed by us has been verified through a recent IR-IR double resonance experiment. However, the scope of such unique structures in protonated oligopeptides and whether protonation at amide oxygen is more stable than that at traditional amino nitrogen remain unsolved. In this study, the most stable conformers of a series of protonated oligopeptides were fully searched. Our findings reveal that the special cis-peptide bond structure appears with high energies for diglycine and is energetically less favored for tetra- and pentapeptides, while it acts as the global minimum only for tripeptides. To explore the formation mechanism of the cis-peptide bond, electrostatic potential analysis, and intramolecular interactions were analyzed. Advanced theoretical calculations confirmed that amino nitrogen is still preferred as the protonated site in most cases except glycylalanylglycine(GAG). The energy difference between the two protonated isomers of GAG is only 0.03 kcal mol^-1, indicating that the tripeptide is most likely to be protonated on the amide oxygen first. We also conducted chemical (infrared (IR)) and electronic (X-ray photoelectron spectra (XPS) and near-edge X-ray absorption fine structure spectra (NEXAFS)) structure calculations of these peptides to identify their notable differences unambiguously. This study thus provides valuable information for exploring the scope of cis-peptide bond conformation and the competition between two different protonated ways.

Theoretical Spectroscopic Studies on Chemical and Electronic Structures of Selenocysteine and Pyrrolysine

The chemical and electronic structures of the 21st and 22nd proteinogenic amino acid selenocysteine (Sec), pyrrolysine (Pyl), and their derivatives (deprotonated and protonated ions) were extensively characterized for the first time. Through the fragment based step-by-step research on their potential energy surface (PES), electronic energies of the most stable conformers of Sec, Pyl and the related ions were finally determined at the advanced CBS-QB3 and DSD-PBEP86-D3(BJ)/aug-cc-pVTZ levels, respectively, with the identification of many new low-energy conformers. The infrared spectra (IR) at 298 K of the most abundant conformers in different forms were scaled by comparison with the anharmonic frequency calculations and analyzed comparing with the experimental spectra of similar molecules. The characteristic soft X-ray spectra (including X-ray photoelectron spectra (XPS) and near-edge X-ray absorption fine-structure spectra (NEXAFS)) of the most stable conformers at 498 K were also simulated. In particular, the two possible protonated configurations of Pyl can be clearly distinguished by their different spectral features. Furthermore, a small binding energy intersection appeared around 293 eV at the C 1s edge between the canonical and protonated Pyl conformers, which is different from all the previous studies. This work thus filled the gap in our knowledge by providing detailed information on the chemical and electronic structures of Sec and Pyl and will be a useful guidance for future experimental research.

Competition between dispersion interactions and conventional hydrogen bonding: insights from a theoretical study on Z-Arg-OH

The dispersion interaction was reported to play a critical role in the stabilization of model dipeptide Z-Arg-OH, even greater than the conventional hydrogen bond (HB), which is opposite to the traditional opinion. Here the conformation of Z-Arg-OH has been systematically searched by the effective fragment based step-by-step strategy. All the newly-found low-energy conformers determined at the advanced DSD-PBEP86-D3(BJ)/aug-cc-pVTZ level are clearly in the stretched form with strong conventional HBs, rather than the reported folded structures with emphasis on the dispersion interactions. The simulated IR spectra of the stretched conformers fit better than those of the folded ones compared with the previous experimental observations. Near-edge X-ray absorption fine-structure (NEXAFS) spectra and X-ray photoelectron spectra (XPS) at C, N and O K-edges have also been simulated to unambiguously identify different isomers. This work thus provides valuable insight into the competitions between the conventional HB and the dispersion interactions and demonstrates that the conventional hydrogen bonding is still more important for such small peptides.

Toward Ab Initio Protein Folding: Inherent Secondary Structure Propensity of Short Peptides from the Bioinformatics and Quantum-Chemical Perspective

By combining bioinformatics with quantum-chemical calculations, we attempt to address quantitatively some of the physical principles underlying protein folding. The former allowed us to identify tripeptide sequences in existing protein three-dimensional structures with a strong preference for either helical or extended structure. The selected representatives of pro-helical and pro-extended sequences were converted into "isolated" tripeptides-capped at N- and C-termini-and these were subjected to an extensive conformational sampling and geometry optimization (typically thousands to tens of thousands of conformers for each tripeptide). For each conformer, the QM(DFT-D3)/COSMO-RS free-energy value was then calculated, G_conf(solv). The Δ G_conf(solv) is expected to provide an objective, unbiased, and quantitatively accurate measure of the conformational preference of the particular tripeptide sequence. It has been shown that irrespective of the helical vs extended preferences of the selected tripeptide sequences in context of the protein, most of the low-energy conformers of isolated tripeptides prefer the R-helical structure. Nevertheless, pro-helical tripeptides show slightly stronger helix preference than their pro-extended counterparts. Furthermore, when the sampling is repeated in the presence of a partner tripeptide to mimic the situation in a β-sheet, pro-extended tripeptides (exemplified by the VIV) show a larger free-energy benefit than pro-helical tripeptides (exemplified by the EAM). This effect is even more pronounced in a hydrophobic solvent, which mimics the less polar parts of a protein. This is in line with our bioinformatic results showing that the majority of pro-extended tripeptides are hydrophobic. The preference for a specific secondary structure by the studied tripeptides is thus governed by the plasticity to adopt to its environment. In addition, we show that most of the "naturally occurring" conformations of tripeptide sequences, i.e., those found in existing three-dimensional protein structures, are within ∼10 kcal·mol^-1 from their global minima. In summary, our "ab initio" data suggest that complex protein structures may start to emerge already at the level of their small oligopeptidic units, which is in line with a hierarchical nature of protein folding.

A random forest learning assisted "divide and conquer" approach for peptide conformation search

Computational determination of peptide conformations is challenging as it is a problem of finding minima in a high-dimensional space. The "divide and conquer" approach is promising for reliably reducing the search space size. A random forest learning model is proposed here to expand the scope of applicability of the "divide and conquer" approach. A random forest classification algorithm is used to characterize the distributions of the backbone φ-ψ units ("words"). A random forest supervised learning model is developed to analyze the combinations of the φ-ψ units ("grammar"). It is found that amino acid residues may be grouped as equivalent "words", while the φ-ψ combinations in low-energy peptide conformations follow a distinct "grammar". The finding of equivalent words empowers the "divide and conquer" method with the flexibility of fragment substitution. The learnt grammar is used to improve the efficiency of the "divide and conquer" method by removing unfavorable φ-ψ combinations without the need of dedicated human effort. The machine learning assisted search method is illustrated by efficiently searching the conformations of GGG/AAA/GGGG/AAAA/GGGGG through assembling the structures of GFG/GFGG. Moreover, the computational cost of the new method is shown to increase rather slowly with the peptide length.

Identification of the smallest peptide with a zwitterion as the global minimum: a first-principles study on arginine-containing peptides

Zwitterions are believed to play an important role in determining the structures, properties and functions of peptides and proteins. However, the smallest peptide with a zwitterionic structure as the global minimum in the gas phase is still not yet identified. In this study, an effective step-by-step strategy has been used to characterize the stable conformers of arginine-containing peptides arginylalanine (ArgAla) and arginylserine (ArgSer). Energy calculations at the DSD-PBEP86-D3BJ/aug-cc-pVTZ level and further extrapolation to the complete basis set (CBS) limit have confirmed, for the first time, that ArgSer appears to be a promising candidate as the smallest peptide with a zwitterionic global minimum structure. First-principles simulations have been performed for near-edge X-ray absorption fine-structure (NEXAFS) spectra and X-ray photoelectron spectra (XPS) at C, N and O K-edges, as well as for infrared (IR) spectra of these arginine-containing peptides. Notable spectral differences were found which enable the unambiguous identification of different neutral forms in future experiments. Our study thus provides valuable insights into the structural stability of zwitterions with the increase of molecular size and illustrates the competition between the canonical and zwitterionic isomers.

Identification of the protonation site of gaseous triglycine: the cis-peptide bond conformation as the global minimum

Extensive ab initio investigations have been performed to characterize stable conformers of protonated triglycine (GGGH) in the gas phase.

Realistic sampling of amino acid geometries for a multipolar polarizable force field

The Quantum Chemical Topological Force Field (QCTFF) uses the machine learning method kriging to map atomic multipole moments to the coordinates of all atoms in the molecular system. It is important that kriging operates on relevant and realistic training sets of molecular geometries. Therefore, we sampled single amino acid geometries directly from protein crystal structures stored in the Protein Databank (PDB). This sampling enhances the conformational realism (in terms of dihedral angles) of the training geometries. However, these geometries can be fraught with inaccurate bond lengths and valence angles due to artefacts of the refinement process of the X-ray diffraction patterns, combined with experimentally invisible hydrogen atoms. This is why we developed a hybrid PDB/nonstationary normal modes (NM) sampling approach called PDB/NM. This method is superior over standard NM sampling, which captures only geometries optimized from the stationary points of single amino acids in the gas phase. Indeed, PDB/NM combines the sampling of relevant dihedral angles with chemically correct local geometries. Geometries sampled using PDB/NM were used to build kriging models for alanine and lysine, and their prediction accuracy was compared to models built from geometries sampled from three other sampling approaches. Bond length variation, as opposed to variation in dihedral angles, puts pressure on prediction accuracy, potentially lowering it. Hence, the larger coverage of dihedral angles of the PDB/NM method does not deteriorate the predictive accuracy of kriging models, compared to the NM sampling around local energetic minima used so far in the development of QCTFF.

Theoretical spectroscopic studies on chemical and electronic structures of arginylglycine

The global minimum of the dipeptide ArgGly is found to be in the canonical form, rather than the zwitterionic form.